Also from “Getting Relief From Light Therapy” in the Fall 2007 Johns Hopkins Depression and Anxiety Bulletin:

Bright-light therapy involves sitting in front of a bank of full-spectrum fluorescent lights for 30-60 minutes each day. To be maximally effective, the light should come from a fluorescent light box that delivers an intensity of 10,000 lux. It is important to position the light box according to your doctor’s (or the manufacturer’s) instructions and to use it the same time each day as consistently as possible. Optimally, bright-light therapy should be used as soon as possible upon awakening and before 10 a.m.

You should really take that last line seriously. In true nutjob fashion, I thought to myself upon receiving my mammoth (largest size available) HappyLite: “If a little bit of light makes you feel good, then a whole lot of light will make you feel even better.” It was the one-case-of-beer-is-better-than-a-six-pack-as-long-as-I-don’t-puke mentality that usually gets me into a bit of trouble.
The night after I got my HappyLite, Eric was away on a golf trip. He would be getting home late, after midnight.
“Perfect!” I thought. “I’ll put the kids to bed, and work under my HappyLite until he gets home.”

I came to the door at 12:30 a.m. my eyes popping out of their sockets as if I had just smoked a crack pipe (not that I know what it looks like …). I lay awake the entire night cursing my HappyLite.
Next doctor’s visit, I told my doctor about my three-hour introduction to light-therapy.
She laughed. “No more than a half-hour at a time, and preferably early in the morning.”
Back to Swartz, the light therapy expert:

Many people respond rapidly to light therapy. Improvement can often be seen within two to four days, with symptoms disappearing within two to three weeks. Continued light therapy throughout the winter season is needed to prevent a relapse, however. If that isn’t feasible, there is some evidence suggesting that relapse can be prevented with the addition of an antidepressant drug after a course of light therapy.
Light therapy should be used only with your doctor’s guidance, as it can cause side effects when used improperly. If you are unable to tolerate the treatment due to agitation, headache, or nausea, your doctor can help you reduce the intensity of the treatment by increasing the distance from the light box or changing the duration of therapy.